1. Field of the Invention
The present invention relates to holograms and holographic optical elements and particularly to such elements used in both the transmission and reflection modes such as in head up display systems.
2. Brief Description of Prior Art
Various forms of holograms, holographic optical elements and holographic systems are well known. The subject matter of the present invention is directed primarily to such devices which are used both in the transmission and reflection modes, such as in head up display systems (HUDs) and, thus, such display systems are of interest with respect to the present invention. Head up display systems generally fall into two categories, conventional optics HUDs and holographic optics HUDs. The purpose of both systems is to allow a pilot or other person in the aircraft to see symbolic representations of important flight data while looking through the windshield or canopy of the aircraft at the real world. In the conventional optics system, one or more partially reflecting flat beam combiners (e.g., partially silvered flat mirrors) are placed between the eyes of the pilot and the windshield, and the pilot can look through it at the real world. An image of the data which is of interest to the pilot is projected by a projection system from a source, such as a cathode ray tube, and the image is reflected from the partially silvered mirror to the eyes of the pilot. In this manner, the pilot can see an image of the data superimposed on what he views out the windshield.
In holographic optics head up display systems, a volume hologram or holographic optical element (HOE) is used in place of the partially silvered mirror. The HOE is used essentially as a transmission element for the real world scene, but it also is used as a reflector for the projected images. It superimposes the real world scene, transmitted through the hologram, with the cathode ray tube image and thus is called a combiner. It provides an improvement over the conventional system in several respects. The holographic optical element can provide a larger field of view with greater reflection efficiency than a partially silvered mirror, can be highly color or wavelength selective while causing little attenuation to transmission of the other wavelengths, and in particular can have optical power and/or aberration correcting capability and provide a collimated image of the display.
Thus, the reflection properties of the hologram are more favorable than the mirror beamsplitter for obtaining high thruput for both display (reflection of a narrow spectral band) and real world views (transmission of a wide, i.e., white light, spectral band minus the narrow band). The optical power available with the hologram allows a wider display field of view than can be obtained using conventional optics of the same size and image quality.
Examples of holographic head up display systems are found in U.S. Pat. Nos. 3,940,204 and 4,261,647. Other literature of interest with respect to holographic optical elements, and the construction and forming thereof are, "Optically Recorded Holographic Optical Elements" by Donald H. Close in Section 10.8 of the Handbook of Optical Holography, Copyright 1979 by Academic Press, Inc.; and articles entitled "Holographic Optical Elements" by D. H. Close, pages 408-419 of Optical Engineering, Volume 14, No. 5, September-October, 1975, "Computer-Originated Hologram Lenses" by R. C. Fairchild and J. R. Fienup, pages 2-14 of SPIE, Volume 215 Recent Advances in Holography (1980), "Using a Conventional Optical Design Program to Design Holographic Optical Elements" by C. W. Chen from pages 15-23 of SPIE, Volume 215 Recent Advances in Holography (1980), "Dichromated Gelatin Holograms and Their Applications" by B. J. Chang, pages 642-648 of Optical Engineering, Volume 19, No. 5, September-October, 1980. The foregoing material provides background information with respect to holographic head up display systems, as well as the manner in which holographic optical elements are constructed and formed, materials used, and processing techniques and the like. As is known, holograms contain fringes, and these fringes normally are parallel to the surfaces for a purely reflective hologram and normally are normal to the surfaces for a purely transmissive hologram.
The aforementioned patents disclose head up display systems using a combiner in the form of one or more holographic optical elements, a projector unit including a generally planar narrow waveband light emitting display surface, such as a cathode ray tube screen, and a suitable relay optical system. The narrow waveband light, such as green light (e.g., having a twenty nanometer bandwidth), from the display screen is transmitted by the optical system to the combiner from which it is reflected to the eyes of the pilot. The image of the display is collimated by virtue of the characteristics of the relay lens and holographic lens, and the holographic lens also transmits the real world scene to the pilot without significant light attenuation.
While holographic optical elements have been used in head up display systems for a number of years, there has existed a persistent problem with many such systems. This problem is one of flare, or spurious or multiple images, that the pilot sees as a result of viewing bright light sources through the holographic element or combiner. This is obviously distracting to the pilot of the aircraft. This problem exists where there is transmission of light through, and reflection of images off of, a high thruput efficiency hologram.